1. Field of the Invention
The present invention relates to a light emitting diode, and more particularly, to a light emitting diode having an active region of a multi quantum well structure.
2. Discussion of the Background
In general, since Group III element nitrides, such as GaN, AlN and InGaN, have excellent thermal stability and a direct transition type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been noticed for its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high density light sources, high resolution output systems and optical communications.
FIG. 1 is a sectional view illustrating a conventional LED having an active region of a multi quantum well structure, and FIG. 2 is a schematic band diagram illustrating the active region of the multi quantum well structure of the LED of FIG. 1.
Referring to FIGS. 1 and 2, an LED comprises an N-type semiconductor layer 17, a P-type semiconductor layer 23 and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 23. In order to increase recombination efficiency of electrons and holes, a P-type clad layer or blocking layer 21 having a relatively wide bandgap may be interposed between the P-type semiconductor layer 23 and the active region 19, and an N-type clad layer (not shown) may be interposed between the N-type semiconductor layer 17 and the active region 19.
The N-type and P-type semiconductor layers comprise Group III element nitride semiconductor layers, i.e., (Al, In, Ga)N-based compound semiconductor layers. The active region 19 is generally formed to have a multiple quantum well structure in which well layers 19a and barrier layers 19b are alternately laminated. In an InGaN LED, an active region of a multiple quantum well structure is generally formed by alternately laminating InGaN well layers 19a and GaN barrier layers 19b. The well layer 19a comprises a semiconductor layer with a smaller band gap than the N-type and P-type semiconductor layers 17 and 19 and the barrier layer 19b, thereby providing quantum wells in which electrons and holes are recombined.
Such a Group III element nitride semiconductor layer is grown on a heterogeneous substrate 11 with a hexagonal system structure, such as sapphire or SiC, by a method, such as organic chemical vapor deposition (MOCVD). However, if a Group III element nitride semiconductor layer is grown on the heterogeneous substrate 11, a crack or warpage occurs in the semiconductor layer and dislocation is produced due to the difference of lattice coefficients and thermal expansion coefficients between the semiconductor layer and the substrate.
In order to prevent these problems, a buffer layer is formed on the substrate 11, wherein the buffer layer generally includes a low temperature buffer layer 13 and a high temperature buffer layer 15. The low temperature buffer layer 13 is generally formed of AlxGa1-xN(0≦x≦1) at a temperature of 400 to 800° C. by a method, such as MOCVD. The high temperature buffer layer 15 is then formed on the low temperature buffer layer 13. The high temperature buffer layer 15 is formed of GaN at a temperature of 900 to 1200° C.
The conventional LED employs a multiple quantum well structure, so that the luminous efficiency thereof can be more improved than that of an LED with a single quantum well structure. In addition, the conventional LED employs the P-type clad layer or blocking layer 21, so that the recombination efficiency can be increased. However, since mobility of electrons is about 100 times as high as that of holes, electrons move within the multiple quantum well structure relatively faster than holes, and therefore, positions at which electrons and holes are recombined are concentrated in the vicinity of the P-type clad layer 21. The P-type clad layer 21 is formed of AlGaN having a relatively wide bandgap. Thus, since the P-type clad layer 21 has large lattice mismatch with respect to GaN or InGaN and contains an impurity such as Mg, a large quantity of crystal defects exist between the P-type clad layer 21 and the active region 19. Such crystal defects cause non-light emitting recombination, thereby decreasing luminous efficiency of the LED.
In order to improve luminous efficiency through recombination of electrons and holes as compared with the non-light emitting recombination, a well layer is disposed as the uppermost layer of the multiple quantum well structure, so that the well layer 19a can be in contact with the P-type clad layer 21. Accordingly, it is expected that electrons concentrated in the vicinity of the P-type clad layer 21 are combined with holes in the uppermost one of the well layers 19a, thereby improving luminous efficiency. However, since the lattice mismatch between the uppermost well layer 19a and the P-type clad layer 21 is more increased than that between the barrier layer 19b and the P-type clad layer 21, crystal defects in the vicinity of the P-type clad layer 21 are increased, and therefore, increasing the luminous efficiency is difficult.
FIGS. 3 and 4 are band diagrams illustrating another problem of a conventional LED having an active region of a multi quantum well structure. FIG. 3(a) simply shows a band diagram of respective layers, and FIG. 3(b) shows a band diagram in an equilibrium state. FIG. 4 is a schematic band diagram when a forward voltage is applied to the conventional LED of FIG. 3. Here, three barrier layers and three well layers are alternately laminated for convenience of illustration (it is noted that the positions of N-type and P-type semiconductor layers 17 and 23 are opposite to those of FIG. 2).
Referring to FIG. 3, the LED includes an N-type semiconductor layer 17 and a P-type semiconductor layer 23 and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 23 as described with reference to FIGS. 1 and 2. In order to increase recombination efficiency of electrons and holes, a blocking layer 21 having a relatively wide bandgap is interposed between the P-type semiconductor layer 23 and the active region 19.
Here, since polarization is generated by a piezo electric field in an active region formed of GaN-based layers, bands of the well layers 19a are inclined in a direction different from those of the barrier layers 19b, considering polarization in FIG. 3(b).
Referring to FIG. 4, when a forward voltage VF is applied to the LED, bands of the N-type semiconductor layer 17 are moved up. When a forward bias that is similar to or higher than the bandgap of the P-type semiconductor layer 23 is applied to the LED, the conduction band Ec of the N-type semiconductor layer 17 is positioned higher than that of the P-type semiconductor layer 23. At this time, the closer the barrier layer 19b in the active region is positioned to the N-type semiconductor layer 17, the higher its conduction band is positioned, as shown in this figure. The arrangement of the bands of barrier layers 19b provides driving force for allowing carriers injected from the N-type semiconductor layer 17 not to be subjected to recombination in the active region 19 but to flow into the P-type semiconductor layer, which accordingly causes carrier overflow to occur as shown by a dotted line arrow. The excessive occurrence of such carrier overflow causes the recombination rate of electrons and holes to deteriorate, thereby decreasing luminous efficiency.
Meanwhile, the blocking layer 21 is employed in order to decrease carrier overflow. The blocking layer 21 is formed of a semiconductor having a wider bandgap to prevent carrier overflow. However, the blocking layer 21 formed of the semiconductor having a wider bandgap has increased lattice mismatch with the P-type semiconductor layer 23, and thus, the crystalline quality of the P-type semiconductor layer 23 grown on the blocking layer 21 deteriorates. Therefore, there is a certain limit to preventing carrier overflow using the blocking layer 21.
As applications of LEDs such as general illumination are extended, the forward voltage VF applied to an LED is not limited to a conventional forward voltage of 3V or so but is continuously increased. Therefore, in an LED operated under high voltage (or high current), developing a new technique for preventing carrier overflow besides the blocking layer 21 is required.
Meanwhile, a technique for doping a barrier layer with an n-type impurity is used to reduce polarization generated by a piezo electric field. However, the n-type impurity doping causes a surface of the barrier layer to be rough, whereby the crystalline quality of the barrier and well layers may deteriorate, and the barrier and well layers are prevented from forming with a uniform thickness. Therefore, improving the crystalline quality of the barrier layer while reducing polarization generated by the piezo electric field is required.